A fuel cell system is known in the art which is provided with a fuel cell stack which generates electricity by an electrochemical reaction between a fuel gas and oxidant gas, an oxidant gas feed pipe which is connected to an inlet of an oxidant gas passage which is formed in the fuel cell stack, a turbocompressor which is arranged in the oxidant gas feed pipe and which feeds oxidant gas, a bypass pipe which branches off from the oxidant gas feed pipe downstream of the turbocompressor, a bypass control valve which controls the amount of oxidant gas which is discharged from the turbocompressor and fed to the fuel cell stack and the amount of oxidant gas which is discharged from the turbocompressor and flows into the bypass pipe, a cathode off-gas pipe which is connected to an outlet of the oxidant gas passage, and a cathode pressure control valve which is arranged in the cathode off-gas pipe and which controls the pressure inside of the oxidant gas passage as the cathode pressure.
In general, in the entire region where the operating point of a turbocompressor can fall, which is determined by a pressure ratio and discharge quantity of oxidant gas of the turbocompressor, a nonsurge region where surging does not occur at the turbocompressor is defined at the side where the pressure ratio is low and the discharge quantity of oxidant as is high, while a surge region where surging can occur at the turbocompressor is defined at the side where the pressure ratio is high and the discharge quantity of oxidant gas is small. That is, if the turbocompressor is operated with an operating point falling in the surge region, surging is liable to occur in the turbocompressor.
Therefore, a fuel cell system is known in the art which sets the discharge quantity of oxidant gas of the turbocompressor to an increased quantity larger than the requested quantity of oxidant gas of the fuel cell stack by an excess amount so that the operating point of the turbocompressor falls in the nonsurge region, when the demanded operating point of the turbocompressor falls in the surge region. This known fuel cell system also controls the bypass control valve so that the excess in the oxidant gas which is discharged from the turbocompressor flows into the bypass pipe in order to maintain the amount of oxidant gas which is fed to the fuel cell stack at the requested quantity of oxidant gas, when the demanded operating point of the turbocompressor falls in the surge region (for example, see Japanese Patent Publication No. 2009-123550A). By doing this, the amount of oxidant gas which is fed to the fuel cell stack is maintained at the requested quantity of oxidant gas while surging is prevented from occurring at the turbocompressor.
On the other hand, a fuel cell system is also known in the art in which it is judged if the fuel cell stack is drying up and, when it is judged that the fuel cell stack is drying up, the opening degree of the cathode pressure control valve is reduced so that the cathode pressure is raised to an increased cathode pressure. If the cathode pressure has risen, the amount of moisture which condenses inside the oxidant gas passage becomes greater. Further, if the opening degree of the cathode pressure control valve is made smaller to make the cathode pressure rise, the amount of cathode off-gas which flows out from the oxidant gas passage in the fuel cell stack becomes smaller, so the amount of moisture which flows out to the outside of the fuel cell stack accompanying the cathode off-gas decreases. As a result, the degree of humidity of oxidant gas passage rises and the drying up of the fuel cell stack is eliminated. In this case, the higher the increased cathode pressure, the shorter the time required for eliminating the drying up of the fuel cell stack.